This invention concerns the protection of metal surfaces against corrosion.
According to the invention there is provided a method of protecting a metal surface comprising the steps of coating the surface with vitreous enamel, spraying an atomized protective metal or alloy whilst the enamel is in a hot viscous condition thereby to form a layer of the protective metal or alloy on the enamel, and carrying out a spray peening operation on the layer of protective metal or alloy whilst the protective metal or alloy is in a hot condition.
The invention also provides an article or structure having at least part of a metallic surface thereof coated with a vitreous enamel, the coating of enamel having applied thereto a spray-coated layer of a protective metal or alloy, with penetration of the enamel at the interface between the enamel and the protective metal or alloy, the protective metal or alloy layer being peened.
Corrosion is a major hazard where articles or structures are required to operate for long periods under adverse weather conditions, particularly marine conditions. Typical cases are those of marine oil platforms and ships.
In the case of marine oil platforms the structure can be weakened, particularly near the waterline and in the splash zone. Painting of the structure is not of itself sufficient protection and sacrificial anodes of zinc are frequently used to diminish corrosion in the danger areas. Despite the use of antifouling paint the formation of barnacles causes further problems because it increases the drag on the structure caused by waves and water currents.
Similar problems occur with ships although the liberal use of antifouling paints and the relative simplicity of inspection ease the problem somewhat.
Many other solutions to the problem of corrosion have been tried including cladding with cupro-nickel or other corrosion resistant metals. This has the special advantage of presenting the seawater side of the combination with a corrosion resistant surface which is toxic to barnacles and therefore permanently antifouling. Unfortunately cupro-nickel and steel form a galvanic cell in the presence of sea water leading to severe corrosion of the steel if penetration of the cupro-nickel occurs. An intermediate layer of rubber can be used to prevent such penetration and separate the components of the galvanic cell but to be effective it must be thick and it is therefore expensive and bulky.